Enhanced uplink for WCDMA is currently being standardized within the Third Generation Partnership Project (3GPP). Among the features introduced is fast scheduling and fast hybrid Automatic Retransmission Request (ARQ) with soft combining, both located in the Node B.
Hybrid ARQ with soft combining allows the Node B to rapidly request retransmissions of erroneously received data entities, leading to significantly reduced delays compared to earlier releases of the WCDMA specification where the Radio Network Controller (RNC) is responsible for all retransmissions within the radio access network. Soft combining with hybrid ARQ can also be used to enhance the capacity of the system by deliberately target multiple transmission attempts for each data entity and use the soft combining mechanism in the receiver to accumulate the received energy until the data is successfully decoded. This can be viewed as implicit link adaptation and is not possible in earlier releases of the WCDMA specification due to lack of a soft combining mechanism in these releases. Typically, a small number of transmission attempts, i.e. a low Block Error Rate (BLER) for the initial transmission, reduce the transmission delays at the cost of a decreased system capacity. Similarly, by targeting a larger number of transmission attempts, i.e. a high BLER for the initial transmission attempt, the system capacity is increased at the cost of increased delays. The choice of hybrid ARQ operating points (in terms of the targeted number of transmission attempts) thus depends on the system load and the delay requirements for a particular service. The possibility for retransmission by the Radio Link Control (RLC) layer in the RNC remains with the introduction of hybrid ARQ in the Node B. This is useful in situations when the hybrid ARQ mechanism in the Node B cannot deliver error-free data entities to the RNC.
Fast scheduling denotes the possibility for the Node B to control when a user equipment is transmitting and at what data rate. Data rate and transmission power is closely related and scheduling can thus also be seen as a mechanism to vary the transmission power used by the user equipment for the enhanced uplink traffic on the E-DPDCH. As the power availability in the user equipment at the time of transmission is not known to the Node B, the final selection of data rate has to be performed by the user equipment itself. The Node B only sets an upper limit on the transmission power the UE may use on the E-DPDCH.
Similarly to the uplink in earlier releases of the WCDMA standard, the enhanced uplink uses inner and outer loop power control. The power control mechanism ensures that a user equipment does not transmit with higher power than required for successful delivery of the transmitted data. This ensures stable system operation and efficient radio resource utilization.
The power control mechanism consists of two parts: an inner loop, located in the Node B, and an outer loop, located in the RNC. The inner loop is fast and updates the user equipment transmission power 1500 times per second in order to combat fast fading. This is done by measuring the received Signal to Interference Ratio (SIR), comparing it with a SIR target, and sending a power control command to the user equipment. If the received SIR is below the SIR target, the user equipment is instructed to increase the transmission power and vice versa if the received SIR is above the target the user equipment is instructed to decrease. The inner loop power control operates on the DPCCH. The transmission power of the E-DPDCH is set relative to the DPCCH and depends on the instantaneous data rate on the E-DPDCH.
The outer loop sets the SIR target in the inner loop and uses statistics available to the RNC, e.g. information whether each data entity for a particular UE delivered to the RNC from the Node B is error-free or not. The outer loop is significantly slower than the inner loop and adapts to slow changes in the radio conditions to match the SIR target to the required quality of service in terms of, e.g., BLER or packet delay.
The introduction of a Hybrid ARQ protocol in the Node B requires modifications to the outer loop mechanism compared to previous releases as the hybrid ARQ protocol ideally hides all the error events from the RNC. Solutions to this problem are described in the patent application PCT/SE2004/000541, where different types of statistics on the Hybrid ARQ operation is proposed to be forwarded to the RNC. One possibility is to inform the RNC about the number of transmission attempts required until a packet is successfully received. If the number of attempts indicated to the outer loop mechanism is larger (smaller) than a target value, the SIR target is increased (decreased), resulting in the inner loop requesting a higher (lower) transmission power from the UE.